This invention relates to the precision attachment of an optical fiber.
The attachment of optical fibers with micrometer positioning accuracy is critical in the optical component industry. Positional, as well as angular and rotational accuracy, and the ability to hold the set position throughout Telcordia qualification of the part, are both challenging and critical in the packaging of most high-speed transmitter and receiver communication components, as well as packaging of fibers to lasers, modulators, or other optical components. With optical fibers and photodiode active areas each having diameters of the order of ten micrometers, fiber attach with single-micrometer positioning tolerance is the norm.
There are two basic alignment approaches available: passive alignment and active alignment. Passive alignment involves the precise placement of all optical components prior to the introduction of the optical fiber. One of those components is a fiber holder, often in the form of a V-groove formed from silicon. This holder is also precisely positioned with respect to the photodiode to which the fiber is to be coupled. After all of the components are attached (often using automated pick and place machines), the fiber is then placed in the pre-aligned fiber holder and, without further need for alignment, the fiber is cemented into place. If the components are placed correctly, the light emitted from the end of the fiber will be directed onto the photodlode. This process often times can use epoxy adhesives for the attachment. The disadvantage of the passive fiber attachment approach is that it provides only coarse, ˜10-micrometer positioning accuracy. This accuracy is adequate for slower, (≦2.5 Gbs) photoreceivers, but not so for the higher speed, 10-GBs and 40-GBs photoreceivers.
For higher speed photoreceivers, in which the detector diameter can shrink to less than 10 micrometers, active alignment of the fiber to the detector is desirable. Active alignment is accomplished by introducing a feedback loop between the detector's electrical output and the positioning stage that is holding the optical fiber. For active alignment applications, there is no pre-positioned fiber holder in the assembly. Instead, the fiber is free to move over some small range until the detected signal (usually in the form of electrical current) from the photodiode reaches the satisfactory value and the fiber is then locked (attached) in place. The feedback loop often times is simply the technician moving the XYZ positioning stage until he/she maximizes the current flow from the photodiode. With this approach, it is possible to align an optical fiber with micrometer, or even submicrometer accuracy.
It is desirable that a fiber attachment process be chosen that is easy to control and assures that the fiber will stay precisely positioned throughout the life of the product. This latter point can be challenging, since these kinds of components are subjected to large temperature swings as well as shock and vibration. Epoxies and low-temperature melting solders are not satisfactory, since they creep by as much as a few micrometers over time and temperature variations. For these reasons, only a few processes are in use today in precision fiber attach of high-speed photoreceivers and transmitters. The primary approach used involves a combination of soldering and welding of the optical fiber to a pedestal that is mounted within the photoreceiver module. This approach can provide the requisite positioning accuracy and can hold to this accuracy over the entire suite of Telcordia tests. However, it requires that the fiber be treated (coated) with a Ni/Au jacket near its end, in order to facilitate solder attachment to the pedestal. This treatment of metal to the optical fiber can add considerable expense to the fiber component. Until recently this expense seemed unavoidable, since the metalized fiber was needed in all cases to hermetically seal the fiber to the receiver module during the final packaging steps. Recently, there has been a push towards lowering manufacturing costs for slower photoreceivers (those that are passively aligned) by eliminating the need for metalized fiber and instead hermetically sealing the fiber to the module using a novel material referred to as low-temperature solder glass. Low-temperature solder glasses are materials that have all the attributes of glass (i.e. their hardness, resistance to creep, temperature cyclability, and hermeticity) but they melt around 300° C. For hermetic sealing applications, a doughnut-shaped solder glass preform is inserted into the Kovar fiber tube, along with the optical fiber (that passes through the doughnut hole). Once the module is completely assembled, heat is imparted to the Kovar fiber tube, often from an inductive-type heater. The solder glass preform then melts, wets, and seals the volume between the loose-fitting optical fiber and the inside walls of the Kovar tube. This is a glass-to-metal seal and is impervious to moisture.